1. Field of the Invention
The present invention relates to a method of redrawing a flanged cup. More particularly, the present invention relates to a method of stably producing a draw-redraw-formed can having an excellent dimensional or shape precision, in which the forming load imposed at the redrawing step can be reduced, and troubles caused by an excessive forming load, such as edge breaking, breaking of the coating and formation of a waste thread (enamel hair), are eliminated.
2. Description of the Related Art
In the field of the can-manufacturing industry, the preparation of a seamless can barrel having a large can height by subjecting a metal sheet, especially a coated metal sheet, to draw-redraw forming has been worked from old.
The preparation of this draw-redraw-formed can is performed according to the process comprising holding a preliminarily drawn cup by an annular blank holder (cup-holding member) inserted into the cup and a redrawing die, relatively moving the redrawing die and a redrawing punch arranged coaxially with the blank holder and redrawing die and movably within the blank holder so that the redrawing punch and the redrawing die are engaged with each other, and draw-forming the cup into a deep-draw-formed cup having a diameter smaller than that of the preliminarily drawn cup. Furthermore, there has already been known a process in which at the redrawing step, by using a redrawing die having a sufficiently small radius of curvature and sufficiently increasing the back tension applied by combination of the blank holder and redrawing die, the thickness of the side wall portion of the final cup is reduced by bending elongation (see Japanese Unexamined Patent Publication No. 01-258822).
In draw forming of a metal sheet, in general, in order to perform draw forming completely under a certain blank-holding force-acting condition, prevent formation of wrinkles in the formed cup and obtain a can having an excellent shape or dimensional precision, it is important that draw forming should be carried out so that a flange is left in the formed cup.
However, in the case where a preliminarily drawn cup is redraw-formed into a cup having a smaller diameter, it is necessary that the flange left in the preliminarily drawn cup should be pulled into the blank-holding surface between the blank holder and the redrawing die, and therefore, the forming load is drastically increased at this point. By this increase of the forming load, edge breaking of the material, breaking of the coating and formation of a waste yarn (enamel hair) are caused, resulting in reduction of the yield of cans or the productivity. Furthermore, in the final can barrel, the portion corresponding to the flange of the preliminarily drawn cup is poor in the shape-manifesting property as compared with other portions, and the dimensional precision is often degraded in this portion. Moreover, it has been confirmed that in the portion through which this residual flange passes, a tool or the like is readily damaged.